For many years, aluminum has been the metallurgy of choice for most integrated circuit applications. Aluminum provides low resistivity characteristics that minimize signal propagation delays and hence enhance chip performance. In order to reduce electromigration defects while enhancing conductivity, small percentages of copper (e.g. up to 10%) have been introduced to form aluminum alloys.
The continuing exponential growth in device density has encouraged process designers to consider metallurgies having enhanced conductivity characteristics. Currently, chemical vapor deposited (CVD) tungsten is being featured in many state-of-the-art designs. However, there is a trend toward finding metallurgies that provide even better conductivity than tungsten. One such metallurgy is pure copper.
In both of the above applications, copper presents difficulties when it is to be etched in an anisotropic mode. Conventionally, metallurgies are anisotropically etched in a reactive ion etch (RIE) mode (i.e., the gaseous plasma is held at a low pressure, and the respective electrodes are held at a high DC bias). Copper reaction products have a low volatility in the presence of the common halogen-based reactive specie that are utilized to etch aluminum metallurgies. In general, this problem has been met by decreasing the chamber pressure and increasing the DC bias, such that the removal mechanism is more of a function of physical bombardment than it is a function of reactive ion complexing. However, with increasing device densities (and the attendant reduced groundrules), the technology has become less tolerant of energetic physical processes.
U.S. Pat. No. 4,468,284 (issued 8/28/84 to Nelson and assigned to Psi Star, Inc.) discloses a process for etching the copper component of an Al/Cu metallurgy. An oxidizing component such as NO.sup.+ is added to a conventional Cl-based aluminum RIE. The NO+ causes electron removal from the copper, to produce an intermediate reaction product NOCl. This intermediate product complexes with the AlCl.sub.3 products of the Al RIE, to produce a reaction product CuCl.sub.2 -Al.sub.2 Cl.sub.6 having a low vapor pressure.
Japanese Unexamined Published Patent Application No. 55-098827 discloses another method of etching the copper component of an Al/Cu alloy. A thin, high-purity aluminum layer is disposed beneath the Al/Cu alloy. Residual copper is removed during the course of etching the underlying thin Al layer.
Because both of the above prior art processes rely on an in situ aluminum source to provide the complexing agent, they will not provide good results when used to etch either an Al/Cu alloy having a high (&gt;4%) Cu component (hereinafter referred to as "Cu-rich Al/Cu alloys"), Cu-based alloys, or a pure Cu film. In the caes of Cu-rich Al/Cu alloy, there will be an excessive amount of Cu present toward the end of the etch cycle relative to the remaining Al. There will not be enough Al available for complexing with the remaining Cu. In the case of the Japanese reference, as the Cu component increases the amount of the underlaying thin Al layer exposed to etching decreases. The above-described drawbacks are particularly true in the case of adaptation to etching a Cu-based alloy or a pure Cu film.
Due to the above-mentioned trends in the technology, a need has developed for a dry etching method that can be used to etch Cu-rich Al/Cu alloys, Cu-based alloys, or pure Cu films, as well as other metal films that normally form low volatility reaction products in Cl-based plasmas.